This invention relates to a hip joint prosthesis having an improved femoral fixation stem.
The femoral ball of a total hip joint prosthesis is affixed to the femur by a fixation stem that is received in the medullary cavity of the femur and held in place by a cement. Over a period of years the femur and, of course, the fixation stem are subjected to several tens of thousands of load cycles consisting primarily of compression and bending loads. Reducing the load during such cyclic loading of any structural member is a high priority objective in the design of such a member. Generally, stresses occurring during these load cycles are greatest in the middle third of the stem. There, tension due to flexure is greatest.
It might seem that the problem of reducing stress could be solved quite easily by providing a stem capable of carrying a higher load with less flexure based on the choice of materials used for the stem and the size and geometry. This approach, however, is unsuccessful. When the femoral fixation stem is in place in the medullary cavity of the femur and the composite structure of bone, cement, and stem is intact, greater stiffness of the stem relative to the bone loads the stem more heavily, taking load off of the bone. Since bone remodels according to the load applied to it, reducing the load on the bone results in a decrease in the amount of bone in the composite structure. Aside from the fact that deterioration of the bone is unwanted, a potential vicious circle exists. The decreased amount of bone that results from decreased bone loading also decreases the stiffness afforded by the bone. This increases the relative stiffness of the chosen stem with respect to the diminished bone. This, in turn, further reduces the load on the bone. Hence, the apparent approach to reducing stress results in an accelerating deterioration of the composite bone, cement, and stem structure that secures the ball of the hip joint prosthesis to the upper leg.